I definitely don't have sex for the first time with someone if they're drunk. Yes, they might be drinking on purpose to lose their inhibitions because they want to have sex to begin with. But there is also the possibility that they're not really into it and the alcohol is making them do things they wouldn't normally do.
I don't want to wonder if, when they sober up, they'll regret it. Much easier to make sure everyone is able to give informed consent before the sex happens, rather than after the fact, when they sober up, confirm that they did consent.
So I totally understand your reservations. I would only have sex while drunk with someone I've been in a relationship with for a while, because then I would figure that we would probably be having sex regardless. If we just had a fight, though, then I wouldn't be comfortable with it, as, once again, they might regret it when they sober up.